EP0258692A2

EP0258692A2 - Surgical prothesis

Info

Publication number: EP0258692A2
Application number: EP87111585A
Authority: EP
Inventors: Donald James Casey; David Wei Wang; Peter Kendrick Jarrett
Original assignee: American Cyanamid Co
Current assignee: Smith and Nephew Inc
Priority date: 1986-08-27
Filing date: 1987-08-11
Publication date: 1988-03-09
Anticipated expiration: 2007-08-11
Also published as: KR950008174B1; NO873598L; JPS6389166A; FI873700A0; FI873700A; DK169278B1; ES2054634T3; DE3750061D1; DK446187A; US4781183A; NO172169B; AU599413B2; DK446187D0; IL83538A; EP0258692A3; AU7746487A; KR880002496A; EP0258692B1; DE3750061T2; NO873598D0

Abstract

A bone fixation device is disclosed. The device comprises an absorbable homopolymer of I-lactide or dl-lactide, or a copolymer of I-lactide, and a reinforcement material. The reinforcement material can be a particulate filler of hydro- , xyapatite or a plurality of alumina fibers.

Description

BACKGROUND-OF THE INVENTION

This invention relates to surgical structural elements consisting of bioabsorbable or semi-bioabsorbable composites, as well as to products and applications using these materials. Such surgical structural devices may include plates, screws, nails, pegs, rods, pins and any other device which would require a bioabsorbable material with relatively high rigidity and strength.
The use of internal bone fixation is an established and clinically used technique. The two major types of internal fixation devices are bone plates and intramedullary rods. The particular form intended for this invention is as an internal bone fixation plate.
Bone plates are applicable to a large variety of bones including load bearing as well as non-load bearing bones. Presently, more long bone fixations are done with intramedullary rods. Both of these devices are traditionally made of metal, e.g. stainless steel 316. There are two major disadvantages, however, associated with the presently used metal plates:

1) Metal plates have a modulus approximately one order of magnitude greater than cortical bone.

This mismatch in stiffness is known to cause stress protection induced osteoporosis or os- teopenia. The increase in porosity and decrease in cortical wall thickness results in a weakened bone which is prone to refracture once the bone plate is removed.

2) The metal plates must be removed due to their nonbiodegradable nature coupled with the eventual possibility of corrosion. A second surgical procedure is therefore required which may introduce further complications.

The use of lower stiffness materials such as nonabsorbable composites and tubular steel have been investigated. However, no human trials of these materials are known.
Lower stiffness bone fixation devices have advantages over metal plates. For example the stiffness of a bone plate can be made essentially equal to the modulus of cortical bone. If the low stiffness bone plate is also bioabsorbable, it does not have to be surgically removed. Thus the need for a second surgical procedure is eliminated.
Considerable research has therefore been devoted to the development of low stiffness bone plate materials. The properties which are most desirable in a bone plate are:

1) The bone plate should provide a firm fixation of the broken bone to promote union during the early stages of healing.
2) Once union has occurred, the load which was initially supported. by the bone plate, would be gradually transferred back to the bone. This would induce the formation of stronger more dense bone at the fracture site thus accelerating the healing process.
3) After the bone heals (3 to 6 months after implantation) the bone plate would completely lose its ability to support a load. The bone would then be once again subjected to its normal stresses.

Bioabsorbable materials with an initial modulus and strength at or near those of cortical bone are useful as internal bone fracture fixation devices for load bearing as well as non-load bearing bones.
The completely bioabsorbable or semi-absorbable composites of this invention are superior in mechanical properties and in biological behavior to the stainless steel devices presently used. The mechanical properties of these composites can be tailored to the specific end-use application. The devices of this invention will gradually lose their mechanical properties and will ultimately fully or partially disappear.

SUMMARY OF THE INVENTION

A bioabsorbable device with an adjustable initial modulus which can be set at, above or below the modulus of bone, and which loses properties at a controllable, predictable rate after implantation has been invented. The device may consist of a poly(l-lactide) matrix reinforced with a-alumina fibers (DuPont Fiber FP^TM) or aramid fibers (DuPont Kevlar^TM), or it may consist of a high molecular weight poly (dl-lactide) matrix reinforced with ultra high modulus polyethylene fibers (Allied A-900^TM, Allied Corp., N.J., U.S.A. These three composite systems are examples of semi--absorbable surgical devices.
This invention uses a combination of materials which may consist of a bioabsorbable polymer and a reinforcement fiber (which may or may not be bioabsorbable), or a bioabsorbable polymer and a bioabsorbable filler. The component materials are combined in such a way as to have bending, axial and torsional stiffness and strength suitable for the biomechanical demands placed upon it. The material will, subsequent to implantation, gradually lose both stiffness and strength according to the time frame for which useful properties are required. The material will ultimately be completely or partially absorbed by the body, any residue being both inert in the body and bereft of significant mechanical properties. No surgical procedure to remove the device would be required.
A bone fixation device has been invented. The device comprises an absorbable polymer and the polymer is obtained from the polymerization of 1-lactide, and a reinforcement material.
In one embodiment, the polymer is obtained from the copolymerization of 1-lactide and dl-lactide. In another embodiment the polymer is obtained from the copolymerization of 1-lactide and glycolide. In a further embodiment, the polymer is obtained from the copolymerization of 1-lactide and 1,3-dioxan-2-one.
In yet another embodiment, and in combination with any of the above embodiments, the reinforcement material is a filler. In a specific embodiment, the filler is in particulate form. In a more specific embodiment, the particulate filler is hydroxyapatite. In another specific embodiment, the filler is selected from the group consisting of tricalcium phosphate, hydroxyapatite, and a mixture thereof.
In a still further embodiment, and in combination with any of the above polymer embodiments, the reinforcement material is manufactured from a plurality of fibers. The fiber material is selected from the group consisting of alumina, poly (_E-phenylene terephthalamide), polyethylene terephthalate, and ultra high modulus polyethylene. In a specific embodiment the fiber is poly(p-phenylene terephthalamide). In another specific embodiment, the fiber is polyethylene terephthalate. In a further specific embodiment, the fiber is alumina.
In a more specific embodiment, the fiber is alpha alumina.
An alternative bone fixation device has been invented. The alternative device comprises an absorbable polymer and a reinforcement material manufactured from a plurality of ultra high modulus polyethylene fibers. The absorbable polymer is obtained from the polymerization of dl-lactide.
Another alternative bone fixation device has been invented. The device comprises an absorbable polymer, said polymer obtained from the copolymerization of 1-lactide, dl-lactide, and a monomer selected from the group consisting of glycolide, 1, 3-dioxan-2-one, and p-dioxanone and a reinforcement material. In one embodiment, the monomer is glycolide. In another embodiment, the monomer is 1,3-dioxan--2-one.
In yet another embodiment, and in combination with any of the above alternative bone fixation device embodiments, the reinforcement material is a filler. In a specific embodiment, the filler is in particulate form. In a more specific embodiment, the particulate fiber is hydroxyapatite. In another specific embodiment, the filler is selected from the group consisting of tricalcium phosphate, hydroxyapatite, and a mixture thereof.
In a still further embodiment, and in combination with any of the above (alternative device) polymer embodiments, the reinforcement material is manufactured from a plurality of fibers selected from the group consisting of alumina, poly(p-phenylene terephthalamide), polyethylene terephthalate, and ultra high modulus polyethylene. In a specific embodiment, the fiber is alumina. In a more specific embodiment, the fiber is alpha alumina.
A laminated bone fixation device has also been invented. The device comprises an impregnating agent consisting of an absorbable polymer matrix. The polymer is obtained from the polymerization of 1-lactide. The matrix has an inherent viscosity of about 1.5 to 3.5 dl/g (0.5 g/dl in CHC1₃). The device also comprises a nonabsorbable reinforcement material. The reinforcement material consists essentially of at least one alumina fiber. The device has a flexural strength of about 10,000 to 25,000 psi; a flexural modulus of about I_X10⁶ to 5x10⁶ psi; a loss of about 30% of initial flexural strength during 3 months in vivo and 60% during 6 months in vivo; and a loss of about 25% of initial flexural modulus during 3 months in vivo and 45% during 6 months in vivo.
In one embodiment, the reinforcement material is a plurality of alpha alumina fibers. In a specific embodiment, the reinforcement material comprises about 10 to 60 volume percent of said fibers. In a more specific embodiment, the device comprises about 15 to 40 percent of said fibers. In another embodiment, the device has a flexural strength of about 15,000 to 25,000 psi. In a further embodiment, the device has a flexural modulus of up to about 3x106 psi.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing the in vivo flexural modulus degradation of the device of this invention, as contrasted with the moduli of prior art bone fixation devices;
Figures 2 and 3 are graphs, similar to that of Figure 1, showing in vivo flexural strength and molecular weight degradation, respectively;
Figure 4 is a top view of a bone fixation device manufactured from the composite materials of this invention;
Figure 5 is a partially broken side view of Figure 4;
Figure 6 is a front view of Figure 5; and
Figure 7 is a broken perspective view of Figures 4 to 6 showing the use of the device on a mammalian bone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following is a description for making the composite materials used in the bone fixation device of this invention.

1. Bioabsorbable Particulate Filled Systems

A) A monomer or monomer mixture is polymerized in bulk in a stirred reactor under nitrogen or vacuum. When the polymer melt viscosity reaches a maximum the particulate filler is added slowly to the concentration desired.
B) A bioabsorbable matrix polymer is heated to melting under nitrogen or vacuum in a mixing chamber. To the melt, the particulate filler (tricalcium phosphate or hydroxyapatite) is added slowly until thorough mixing is observed at the desired concentration.

2. Fiber Reinforced Systems

A) Solution Impregnation and Laminations: The fiber or woven fabric is immersed in a solution of the biodegradable polymer in a low boiling point solvent (e.g. methylene chloride). The amount of polymer deposited on the fabric, chopped fiber or fiber yarn is dependent on the solution concentration, the polymer molecular weight (which effects solution viscosity) , the length of immersion time and the number of immersions. The impregnated chopped fiber, yarn or fabric (prepreg) is then thoroughly dried. The prepreg is laid-up in a mold of a predetermined thickness. Vacuum is applied to the lay-up by use of a vacuum bag. Heat and compression are then applied to consolidate the laminate.
B) Melt Impregnation and Lamination: Films of the biodegradable polymer are made by solvent casting or melt pressing. Alternatively, fibrous mats are made from polymer by running a solution of the polymer into a non-solvent in a thin stream to form a stringy precipitate, followed by pressing into a mat at room temperature. The films or mats are then laid between yarn or fabric layers in a mold of a predetermined thickness. Vacuum is applied to the lay-up, by vacuum-bagging the mold, and heat and compression are applied to consolidate the laminate.

Figures 4 to 6 show the bone fixation device. The device can be manufactured without undue experimentation by methods known in the prior art, e.g. by compression molding. The device can be attached to a bone by any means presently known or obvious to a person having ordinary skill in the art, for example by fastening with screws, staples and the like, or by bonding, for example, by gluing the device to the bone.
Figures 4 to 6 show holes 2 which are used to each accommodate a screw 5 (shown in Figure 7). To accommodate the screw head, a plurality of holes 2 are countersunk 3 in the device 1.
Referring specifically to Figures 4 and 5, four holes 2 are shown. It is to be understood that any number of holes can be used, provided the device 1 is adequately attached to a bone 4 (shown in Figure 7). However, as a minimum, at least two holes 2 and screws 5 appear to be necessary.
Referring to Figure 7, the preferred relationship of the device 1 to a mammalian bone 4 fracture is shown. Under many circumstances, the configuration shown in Figure 7 will allow the best possible chance for uniform healing of the fracture in the bone 4.
The following examples more fully describe the above embodiments.

Example 1

Poly(1-lactide)-Alumina Fiber Laminate: The laminate was formed from poly(l-lactide) of inherent viscosity 2.68 dl/g (0.5 g/dl in CHC1₃, after consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 4" by 4" square films, two films of 0.005" and two films of 0.023" thickness. The laminate was formed by stacking the films and fabric in alternating layers. Three plies of fabric were used. The 0.005" thick films were used for the -two outside layers. The laminate was consolidated by heating to 200⁰C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 19% alumina fabric by volume. The laminate had the following mechanical properties:
This material is of particular interest due to it's superior in vivo performance in subcutaneous rabbit implant experiments. The degradation of this material in vivo is shown in Figures 1-3. Figures 1 and 2 show the physical property degradation profile and Figure 3 shows the molecular weight degradation profile, as contrasted with materials used in prior art bone fixation devices. The initial properties of this composite system can be varied over a wide range by varying fiber loading. In addition, the degradation profile can be altered by varying the initial molecular weight of the matrix polymer.
Mechanical properties declined in a roughly linear manner over a 6 month period in rabbits to a level at 39% of initial flexural strength and 53% of initial flexural modulus. Inherent viscosity data suggest that mass loss of the poly(l-lactide) matrix would begin after approximately 42 weeks. After mass loss onset, the rate of mechanical property degradation should increase and any remaining load bearing capability would quickly deteriorate.

Example 2

Poly(1-lactide)-Kevlar Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 1.00 dl/g (0.5 g/dl in CHCl₃, before consolidation) and a satin weave Kevlar 49 fabric. The polymer was dissolved in methylene chloride at a concentration of 5% (w/v). Kevlar fabric was immersed in the solution to form a prepreg of 22% poly(l-lactide) by weight. Poly(l-lactide) was melt pressed into films approximately 0.004" thick. Seven polymer films and six plies of prepreg were laid-up in alternating layers. The laminate was consolidated by heating at 200°C in a vacuum bag and compressing to a thickness of 1/16". The resulting laminate was 49% Kevlar by volume. The laminate had the following mechanical properties:

Example 3

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(1-lactide) of inherent viscosity 1.64 dl/g (0.5 g/dl in CHC1₃, after consolidation) and a fabric made from alumina fiber. Poly(1-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 4" by 4" square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers. Three plies of fabric were used. The 1.2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties:

Example 4

Poly(1-lactidey-Alumina Laminate: A laminate was formed which consisted of a poly(1-lactide) of inherent viscosity 2.65 dl/g (0.5 g/dl in CHC1₃, after consolidation) and a fabric made from alumina fiber. Poly(1-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 4" by 4" square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers. Three plies of fabric were used. The 1.2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195°C in a vacuum bag and compressing to a-thickness of 1/16". The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties:

Example 5

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 4.14 dl/g (0.5 g/dl in CHC1₃, after consolidation) and a fabric made from alumina fiber. _Poly(l-lactide) was reprecipitated from a chloroform solution into methanol. The dried precipitate was pressed into 4" by 4" square mats, two mats of 6.5 g and two mats of 1.2 g. The laminate was formed by stacking the mats and fabric in alternating layers. Three plies of fabric were used. The 1.2 g mats were used for the two outside layers. The laminate was consolidated by heating to 195⁰C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 17% alumina fabric by volume. The laminate had the following mechanical properties:
A summary of the flexural strength and flexural modulus data for Examples 3 to 5 is contained in the following Table.

Example 6

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHC1₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 4" by 4" square films, two films of 0.005" and one film of 0.045" thickness. The laminate was formed by stacking the films and fabric in alternating layers. Two plies of fabric were used. The 0.005" films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 13% alumina fabric by volume. The laminate had the following mechanical properties:

Example 7

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHC13, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 4" by 4" square films, two films of 0.005" and two films of 0.023" thickness. The laminate was formed by stacking the films and fabric in alternating layers. Three plies of fabric were used. The 0.005" films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 19% alumina fabric by volume. The laminate had the following mechanical properties:

Example 8

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHC1₃, before consolidation) and a fabric made from alumina fiber. Poly(1-lactide) was melt pressed into 4" by 4" square films, two films of 0.005" and three films of 0.015" thickness. The laminate was formed by stacking the films and fabric in alternating layers. Four plies of fabric were used. The 0.005" films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 24% alumina fabric by volume. The laminate had the following mechanical properties:

Example 9

Poly(1-lactide)-Alumina Laminate: A laminate was formed which consisted of a poly(l-lactide) of inherent viscosity 2.64 dl/g (0.5 g/dl in CHCl₃, before consolidation) and a fabric made from alumina fiber. Poly(l-lactide) was melt pressed into 4" by 4" square films, two films of 0.005" a.nd four films of 0.008" thickness. The laminate was formed by stacking the films and fabric in alternating layers. Five plies of fabric were used. The 0.005" films were used for the two outside layers. The laminate was consolidated by heating to 200°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 30% alumina fabric by volume. The laminate had the following mechanical properties:
A summary of the flexural strength and flexural modulus data for Examples 6 to 9 is contained in the following Table.

Example 10

Poly(1-lactide)-Alumina Laminate: A laminate was formed by impregnating 1/2" chopped alumina fiber with poly(l-lactide). The polymer had an inherent viscosity of 2.64 dl/g (0.5 g/dl in CHC1₃, before consolidation). The impregnation was accomplished by dissolving the polymer in chloroform (10 g/dl) followed by stirring in the chopped fiber. The mixture was then dried under vacuum to constant weight. The impregnated fiber was consolidated using vacuum and compression at 200°C, forming a laminate containing 30% alumina by volume. The laminate had the following mechanical properties:

Exampl*e 11

In Vitro Degradation of Poly(l-lactide)-Alumina Laminates: An accelerated in vitro degradation test was used to assess the relative degradation rates of laminates made with poly(1-lactide)s of different molecular weights reinforced with alumina fabric. The in vitro procedure involved immersing the sample in a pH 6.09 phosphate buffered aqueous solution at 67°C. The samples were removed from the bath, dried and tested for mechanical properties using the ASTM D790 method. Samples from Examples 3, 4 and 5 were used in this study. The results are shown in Table I. These data indicate that the composite fabricated with the lower molecular weight polymer (Example 3) possessed higher initial mechanical properties than the composites made with higher molecular weight polymers. It also appeared to have less scatter in its degradation profile.

Example 12

In Vitro Degradation of Poly(1-lactide)-Alumina Laminates: An accelerated in vitro degradation test was used to assess the relative degradation rates of laminates made with poly(l-lactide) reinforced with different loadings of alumina fabric. The in vitro procedure was identical to that described in Example 11. Samples from Examples 6, 7, 8 and 9 were used in this study. The results are shown in Table II. These data indicate that the composites possessed higher initial mechanical properties as the fabric volume increased. This relationship allows the tailoring of a material to have the mechanical properties desirable for a specific application within a fairly broad range.

Example 13

Poly(dl-lactide)-Polyethylene Laminate: A laminate was constructed using Ultra High Modulus Polyethylene (UHMPE) and poly(dl-lactide). The UHMPE fiber was laid-up in unidirectional plies with 0°, 90° orientation. Between each ply, a 0.003" thick film (melt pressed) of poly(dl-lactide) was laid. A film of polymer was placed on the top and the bottom of the lay-up as well. The laminate was consolidated by heating to 120°C in a vacuum bag and compressing to a thickness of 1/16". The laminate contained 41% UHMPE by volume. The laminate had the following mechanical properties:

Example 14

Poly(1-lactide)-Polyethylene Terephthalate Laminate: Poly(l-lactide) with an initial inherent viscosity of 3.63 dl/g (0.5 g/dl in CHC1₃) was dissolved in CHC1_3/ethyl acetate (V/V 9/1), at a concentration of 1_0% (w/v). Polyethylene terephthalate fabric was impregnated by dipping in the solution to a coating level of~50% by weight. Six plies of this prepreg were then consolidated in a heated hydraulic press at 180^*C for 3 minutes with about 1500 psi pressure. The resulting laminate had a flexural modulus of 0.43 X 10⁶ psi.

Example 15

Poly(l-lactide)-Hydroxyapatite Composite: Poly-(1-lactide) was prepared by charging 100 g of 1-lactide, 15.5 ul (0.01 mole %) lauryl alcohol and 15.6 mg (0.01 mole %) stannous chloride dihydrate into a stirred reactor at 200°C. When the power drain on the stirring motor reached a maximum, 45 g of hydroxyapatite (Ca₁₀(OH)₂(PO₄)₆, Mallinckrodt) was added. The composite was discharged after it appeared homogeneous (about 5 min.). The composite contained about 14% hydroxyapatite by volume. The flexural properties of a compression molded plaque were:

Claims

1. A bone fixation device comprising an absorbable polymer, said polymer obtained from the polymerization of 1- lactide, and a reinforcement material.

2. A device of claim 1 wherein said polymer is obtained from the copolymerization of 1-lactide and dl-lactide.

3. A device of claim 1 wherein said polymer is obtained from the copolymerization of 1-lactide and glycolide.

4. A device of claim 1 wherein said polymer is obtained from the copolymerization of 1-lactide and -1,3-dioxan-2-one.

5. A device of claim 1 or 2 or 3 or 4 wherein the reinforcement material is a filler.

6. A device of claim 5 wherein the filler is selected from the group consisting of tricalcium phosphate, hydroxyapatite, and a mixture thereof.

7. A device of claim 1 or 2 or 3 or 4 wherein the reinforcement material is manufactured from a plurality of fibers selected from the group consisting of alumina, poly-(p-phenylene terephthalamide), polyethylene terephthalate and ultra high modulus polyethylene.

8. A device of claim 1 or 2 or 3 or 4 wherein said polymer has an inherent viscosity of about 1.5 to 3.5 dl/g (0.5 g/dl in CHC1₃).

9. A device of claim 8 wherein said reinforcement material consists essentially of at least one alumina fiber, said device having a flexural strength of about 10,000 to 25,000 psi; a flexural modulus of about 1x10⁶ to 5x10⁶ psi; a loss of about 30% of its initial flexural strength during 3 months in vivo and 60% during 6 months in vivo; and a loss of about 25% of its initial flexural modulus during 3 months in vivo and 45% during 6 months in vivo.

10. A device of claim 9 comprising about 10 to 60 volume percent of said alumina fibers.